Boot up with the first disk and press F1 and enter the option "rescue".
Select the Mount Partititions on /mnt.
At this point, you are the root user.
Change the root dir to /mnt:
Note, If as a normal user, you can run: init 1
this may be quicker.
Now you need to delete the old encripted password from the /etc/shadow entry.
Here is the rest of the instructions copied and pasted from a mandrakeuser.org howto. Note that if you are using the rescue mode, you may need to use vim-minimal rather then vim.
Next, run vi /etc/shadow.
The format of entries in '/etc/shadow' is
account_name:password:other stuff e.g.
The password entry is encrypted, of course.
Delete the password entry for 'root' by moving the cursor to first character of the password (usually the first '$') and typing dw. Now type :wq to save the file.
Reboot with init 6
On the next login, type root for the account name and just hit <ENTER> at the 'password' prompt.
Once you are logged in, type passwd to give 'root' a new password.
I hope this helps.